1. Field of the Invention
The present invention relates to a shoe press belt for manufacturing paper, and more particularly to a closed type shoe press belt and a manufacturing method therefor.
2. Discussion of the Prior Art
Shoe presses, which are used in the pressing unit of a paper making machine, are typically of two types: an open type and a closed type. Of these types, the closed type shoe press is mainly used in recent years because the open type shoe press requires a larger installation space and potentially harmful oil diffusion has been associated with it.
FIG. 7 shows a closed type shoe press. Referring to FIG. 7, numeral 11 denotes a press roll, numeral 12 denotes a shoe, and numeral 13 denotes a shoe press belt. The shoe press belt 13 is formed endlessly, and travels together with a top felt 15, a wet (paper) web 14 and a bottom felt 15' so that the wet web is wrung dry by pressurization between the press roll 11 and the shoe 12.
The shoe press belt 13 is driven through the felt 15, the wet web 14 and the felt 15' by the press roll 11 as the driving source. More specifically, when the shoe press belt 13 travels in the leftward direction indicated by the arrow in FIG. 8, a pressurized portion (the cross-hatched portion) and a non-pressurized portion (the portions which are not cross-hatched) occur at both ends of the shoe 12 in the CMD direction (FIG. 1) at the boundary between the pressurized and un-pressurized portions. A driving force is exerted on the pressurized portion, while both end portions, which are not pressurized, lag. Therefore, the shoe press belt 13 is subjected to stress S in an oblique direction (in the bias direction indicated by the arrows S in FIG. 8) in the end portions of the belt, causing distortion which results in cracks.
Moreover, since the conditions of use are severe owing to a short length of circumference (dimension) of the shoe press belt, durability of the belt is highly desirable, and various measures are taken to make it durable in its structure and method of manufacture.
Typical such measures are described in Japanese Patent Publication No.3-57236, Japanese Laid-Open Patent Application No.64-45889, Japanese Translation of Unexamined PCT appln. No.1-503315, Japanese Laid-Open Patent Application No.1-298292 and Japanese Translation of Unexamined PCT appln. 5-505428.
According to the Japanese Patent Publication No.3-57236, however, the base fabric of the shoe press belt is installed around a mandrel and resin is poured onto the fabric. This is performed by using the so-called die forming method, which to its merit that uniform thickness and surface smoothness can be obtained, but has a problem that it is difficult to uniformly set the position of the base fabric in the width (i.e., CMD) direction.
According to the Japanese Laid-Open Patent Application No.64-45889, after an uncured resin sheet is installed around a mandrel, the mandrel is covered with base fabric on top of the resin sheet, is further wound with a second uncured resin sheet, and is finally wound with a ribbon consisting of heat-shrinkable resin to be thermally shrunken. In this case, since the base consists of fabric, it is difficult to dimensionally match circumferentially, which may possibly cause pinholes.
According to the Japanese Translation of Unexamined PCT appln. No. 1-503315, no fabric is used as the base, but thread is stretched over the overall circumference at predetermined intervals in the axial direction (CMD direction) of the mandrel, on top of which thread is spirally wound in the circumferential direction (MD direction), and resin is supplied for formation of the belt. This method has to its merit that the change in dimension due to tension in the CMD direction is small, but has a difficulty in that it takes time to manufacture.
Further, according to the Japanese Translation of Unexamined PCT appln. No. 1-298292, a mat-shaped fibrous belt is impregnated with uncured resin, and is spirally wound around the mandrel to be heated and cured. This shoe press belt has a problem with dimensional stability in both the MD direction and the CMD direction, and in addition, it peels.
Further, according to the Japanese Translation of Unexamined PCT appln. No.5-505428, fabric made of thread with a low elasticity modulus is used in the MD direction and thread with a higher elasticity modulus than the MD directed thread is used in the CMD direction. The fabric is impregnated with uncured resin, and is spirally wound around the mandrel to be cured. This shoe press belt is subject to peeling when a stress is applied to the overlapped portion of the wound fabric.